1. Field of Invention
This invention pertains to a process for growing single crystal chalcogenide materials or compounds of one or more divalent metals, with or without a transition element. The divalent metals are selected from alkaline earths, Group 12 elements, and europium.
2. Description of Prior Art
Single crystals of CaS, with no added impurity, have been grown by vapor phase transport using iodine. Typically, the charge of powdered CaS was located at one end of a silica ampoule of about 200 mm in length and 28 mm bore. After gently heating in vacuum with about 1 .mu.g/mm.sup.3 of iodine added to the charge, the ampoule was sealed under vacuum and then placed in a furnace with the CaS charge at about 1130.degree. C. and the growth end of the ampoule at about 830.degree. C.
Single crystals of alkaline earth chalcogenides have been prepared by the floating zone method using a Xe-arc image furnace. The starting sulfide powder materials were prepared by heating the respective carbonates at 1100.degree. C. in a gas stream mixture of H.sub.2 S/H.sub.2. The products were pulverized to fine powders and the procedure was repeated to obtain final materials. The powders were then stuffed into a thin rubber tube forming a rigid rod 3-6 mm wide and 100-150 mm long. The tube was then immersed in an oil cylinder of an oil press and was compressed under a hydrostatic pressure of about 1000 kg/mm.sup.3. After peeling off the rubber tube, a rod of compressed powder was obtained which was sufficiantly stiff without any binder or subsequent sintering.
Using the Kitazawa et al. Xe-arc image furnace, a first piece of the pressed rod was mounted at bottom therein to serve as a seed for single crystal formation and a second piece of the pressed rod was mounted at top of the apparatus spaced from but coaxial with the first piece. Discharge of the Xe lamp was focussed at the top of the seed rod, thus forming a hot spot. The second piece was slowly lowered towards the first while the discharge current of the lamp was increased until the tip of the second rod was melted to form a melted drop of the second piece of the rod. At this point, lowering of the second piece was stopped and the first piece was raised towards the hot spot to form a floating melt zone when the melted drop of the second piece was fused with the first piece in the hot spot. Then the lower part of the floating melt zone was slowly pulled down out of the hot spot and was solidified into crystalline form. By sweeping the melt zone across the second rod piece on the feed rod, growth of the single crystal took place along the &lt;100&gt; direction. Seed crystals from this process can be used to grow single crystals of larger size.
Polycrystalline samples of the series Ca.sub.1-X Y.sub.2x/3 S, with x=0.2-0.7, were prepared by reacting mixtures of stoichiometric quantities of the binary sulfides at 1025.degree. C. for 10 days and then slowly cooling to encourage cation/vacancy ordering. Reactions were carried out in carbon crucibles enclosed in evacuated silica ampoules that had been previously outgassed under vacuum at 900.degree. C. Single crystals were grown from eutectic CaCl.sub.2 :KCl flux and 1:1 molar mixture of the binary sulfides. The reaction mixture consisting of charge and flux at respective weight ratio of 1:6.5 was heated at 1025.degree. C. for 3 weeks and then slowly cooled to room temperature.
Crystals of CaY.sub.2 S.sub.4, SrY.sub.2 S.sub.4 and BaY.sub.2 S.sub.4 were grown from eutectic halide fluxes having melting points in the range of about 620-645.degree. C. Synthesis was carried out in carbon crucibles enclosed in silica ampoules evacuated to less than 10.sup.-4 Torr that were previously outgassed under vacuum at about 900.degree. C. Exposure of the reaction mixtures to moisture was minimized and the fluxes were stored in a vacuum oven at 140.degree. C. For crystal growth of CaY.sub.2 S.sub.4 and SrY.sub.2 S.sub.4 crystals, the charges were 1:1 molar mixtures of the respective binary sulfides whereas for BaY.sub.2 S.sub.4 crystals, the polycrystalline form of the compound was used, the compound being prepared by reacting a stoichiometric mixture of the binary sulfides at 1000.degree. C. for 1 month. The crystals were obtained by heating about 1:10 charge:flux mixture over a period exceeding 10 hours to above 1000.degree. C., soaking for up to 20 days and then slowly cooling. The crystals were separated from the flux by soaking in distilled water.